Degradation of cellular mRNAs during Kaposi’s sarcoma-asso-ciated herpesvirus infection is assosarcoma-asso-ciated with hyperadenylation of transcripts and a relocalization of cytoplasmi
Trang 1Degradation of cellular mRNAs during Kaposi’s
sarcoma-asso-ciated herpesvirus infection is assosarcoma-asso-ciated with hyperadenylation
of transcripts and a relocalization of cytoplasmic poly(A)-binding
proteins to the nucleus
The cellular machinery for RNA decay plays a major role in
regulating gene expression and as a mechanism for RNA
quality control [1] Increasing evidence suggests that
viruses have evolved ways of interfacing with the cellular
RNA decay machinery that aid their survival and replica
tion First, viral transcripts must avoid degradation if they
are to be effectively translated Second, viruses often
induce the degradation of cellular mRNAs, which gives
their own transcripts a competitive edge for access to the
cellular translation machinery The mechanisms under
lying these strategies are currently being elucidated In
addition to providing a clearer understanding of virushost
interactions, the mechanisms used by viruses to usurp the
cellular RNA decay machinery may also provide insight
into innate cellular mechanisms This point is well
illustrated in a recent paper in PLoS Biology by Yeon Lee
and Britt Glaunsinger [2] on a novel RNA decay mecha
nism induced by Kaposi’s sarcomaassociated herpesvirus
(KSHV) Kaposi’s sarcoma is the most common tumor in
people with AIDS and results from chronic infection with
the virus However, like other herpesviruses, KSHV causes
a lytic infection when reactivated and during this phase
shuts off hostcell functions by inducing a global
destruction of mRNA
KSHV-encoded SOX protein induces mRNA
decay
KSHV initiates global decay of cellular mRNAs via
expression of the virusencoded ShutOff and Exonuclease
(SOX) protein [3] Unlike the virion shutoff protein (VHS)
of the related herpes simplex virus [4], SOX itself does not
possess any demonstrable nuclease activity [5], and so how
it induces mRNA decay is of considerable interest In
addition, bioinformatic analyses fail to identify any
proteinprotein interaction domain that would provide a
clue to possible coeffectors of SOXinduced mRNA
degradation Thus, Lee and Glaunsinger [2] had relatively
little to guide them as they set out to define the mechanism
of SOXinduced RNA decay
Through a careful analysis of mRNA modifications, locali
za tion, and RNAbinding proteins during SOXinduced mRNA degradation, Lee and Glaunsinger made four key observations using a series of transfections and viral infections in human 293T and TIME (telomeraseimmor talized microvascular endothelial) cells First, they documented a clear increase in the size of the poly(A) tail
of target RNAs in the presence of SOX that correlated with
a decrease in the relative stability of the transcripts Presumably this is due to the addition of adenosines, although other nucleotides cannot formally be ruled out [6] Second, PAPII, the major poly(A) polymerase in the cell that is responsible for the initial mRNA poly adeny lation event, was required for this hyperadenylation This suggests that the PAPII is involved in the hyperadenylation, although it is not entirely clear whether its role is simply to provide the poly(A) tail to be extended or if it is directly responsible for adding the extra 3’ nucleotides Another protein that influences the primary polyadenylation event, the nuclear poly(A)binding protein PABPN1 [7], is also required for SOXmediated mRNA hyperadenylation and decay Third, there was a dramatic increase in poly(A)+
RNAs in the nucleus, suggesting that the hyperadenylation occurred on many different mRNAs and that an mRNA trafficking pathway was probably being affected Fourth, in the presence of SOX, the cytoplasmic poly(A)binding protein PABPC1 was dramatically relocalized to the nucleus A similar relocalization of PABPC1 to the nucleus has also been observed in patientderived KSHVinfected cell lines [8] Movement of PABPC1 to the nucleus was directly correlated with the ability of SOX protein to induce decay of cytoplasmic RNAs Furthermore, knockdowns of PABPC1 by RNA interference (RNAi) reduced the ability of SOX to induce RNA turnover Finally, reporter mRNAs (made using ribozyme technology) that lacked a 3’ poly(A) were immune to SOXmediated RNA degradation, directly correlating hyperadenylation with SOXmediated decay Interestingly, histone mRNAs that naturally lack a poly(A) tail can still be degraded in a SOXdependent fashion even though they are not hyperadenylated Thus, whereas the bulk of mRNA decay mediated by SOX involves
Kevin J Sokoloski, Emily L Chaskey and Jeffrey Wilusz
Address: Department of Microbiology, Immunology and Pathology, Colorado State University, Fort Collins, CO 80523, USA
Correspondence: Jeffrey Wilusz Email: jeffrey.wilusz@colostate.edu
Trang 2hyperadenylation and PABPC1 relocalization, alternative
degradation pathways appear to exist
Because hyperadenylation of RNAs has been associated
with nuclear surveillance for RNA quality in yeast [9,10],
and to a lesser extent in mammals [11,12], an attractive
hypothesis is that SOX is causing the cell’s quality control/
RNA surveillance machinery to degrade normal mRNAs in
some fashion, perhaps by reorganizing the structure of
messenger RNA ribonucleoprotein (mRNP) particles
Although this idea is consistent with the PABPC1 relocali
zation to the nucleus, it should be emphasized that it is
currently unclear whether this relocalization is a cause, or
a consequence, of SOXinduced RNA degradation The
SOX protein does not possess known interaction domains
for poly(A)binding proteins (for example, PAM2 [13]), nor
do SOX and PABPC1 coimmunoprecipitate Thus, SOX is
likely to modulate PABPC1 localization via an indirect
mechanism
Curtailing the actions of poly(A)-binding
proteins is a common viral strategy
Poly(A)binding proteins have a multitude of functions in
the cell, including the stimulation of polyadenylation, the
nuclear export of mature mRNAs, regulation of translation
efficiency and an influence on mRNA decay [14] They
therefore make an attractive target for viruses, as
interfering with poly(A)binding function would have a
ripple effect on gene expression throughout the cell In
fact, as outlined in Figure 1, numerous RNA viruses, inclu
ding picornaviruses, caliciviruses, HIV, rotavirus, rubella
virus and now KSHV, have evolved strategies to interfere
with this function These include the cleavage, subcellular
relocalization and binding/sequestration of poly(A)
binding proteins, as well as the inclusion of binding sites
for poly(A)binding proteins in the viral genome that are
not adenosine tracts all of which could interfere with the
normal function of poly(A)binding proteins In the absence
of functional PABPC1, viruses would naturally have to develop
a mechanism for maintaining the stability and translatability
of their mRNAs; this is achieved in some viruses by the
presence of internal ribosome entry sites (IRES) [15]
Curiously, KSHV appears to lack IRES elements
Unanswered questions and future directions
KSHV induces the decay of approximately 95% of the
cellular mRNA during infection, and it is unknown how
sufficient levels of its own viral mRNAs escape SOX
induced mRNA decay to support a productive infection
Bioinformatic analysis of herpesvirus mRNAs fails to
reveal any of the ciselements that commonly mediate SOX
resistance There are at least three ways that KSHV mRNAs
could selectively escape SOXmediated RNA decay First,
they might encode a cisacting element or bind to a trans
acting factor that stabilizes the poly(A) tail and thus
prevents hyperadenylation and/or degradation The nuclear
expression and retention element (ENE) in the KSHV PAN
RNA [16] is a known example of a cisacting element that is
essential for the nuclear accumulation of this RNA A second possibility is that whereas polyadenylation appears
to deposit the protein nucleophosmin near the 3’ end of cellular mRNAs [17], perhaps different proteins (with different downstream effects on mRNA fate) are deposited
as a result of polyadenylation on virally encoded signals Third, because of differences in RNA elements or mRNP structures, viral and cellular mRNAs may interact differ ently with components of the nuclear export machinery, thereby altering mRNA fate Interestingly, KSHV encodes orf57, a protein required for lytic infection that stabilizes and exports the intronless viral mRNAs from the nucleus [18] If a failure to export mRNAs from the nucleus is related to SOXmediated shutoff of cellular RNAs, could this factor be responsible for the SOX resistance of viral RNAs? Undoubtedly, further research will be focused on determining the resistance of viral transcripts to the mislocalization of PABPC1 and the induced mRNA decay
Figure 1
PABPC1 is a common target for viral perturbation of cellular processes RNA viruses have developed a variety of strategies to interfere with or usurp the cytoplasmic poly(A)-binding protein PABPC1 This interference generally shuts down the translation of host-cell mRNAs as well as potentially exposing them to rapid degradation by the RNA decay machinery (a) A variety of
picornaviruses [22], caliciviruses [23] and HIV [24] encode proteases (for example, poliovirus 2A) that specifically cleave PABPC1 (b) The rotavirus nsp3 protein [25], as well as the KSHV
SOX protein [2], relocalizes PABPC1 to the nucleus Interestingly, unlike SOX, nsp3-induced relocalization does not appear to result in increased mRNA decay (c) Rubella virus capsid protein specifically
binds to PABPC1, sequestering the protein and presumably preventing its binding to cellular mRNAs [26] (d) Despite the
absence of a poly(A) tract, sequences in the 3’ untranslated region
of dengue virus can specifically bind PABPC1 [27] and recruit it for use by viral mRNAs ORF, open reading frame
PABP PABP
PABP Viral
capsid PABP
PA BP
(a) Proteolytic cleavage
(b) Mislocalization
(c) Binding and sequestration
Cytoplasm Cytoplasm
ORF PABP
Picornaviruses Caliciviruses HIV
Rotavirus KSHV
Rubella virus
(d) Novel PABP-RNA interactions
Dengue virus
Nucleus NucleusPABP
Trang 3The SOXmediated hyperadenylation of mRNA raises
several interesting questions First, what proteins are
directly responsible for the 3’end mRNA tailing during
KSHV infection? Although PAPII is a strong candidate, the
potential roles of numerous cellular noncanonical
poly(A/U) polymerases [6] has not been tested Further
more, tailing mRNAs with uridines rather than adenosines
has been shown to activate the decay of mammalian
histone mRNAs [19] and certain transcripts in Schizo
saccharomyces pombe [20] Second, although it is
assumed that hyperadenylation probably sets up a plat
form for exonucleases (as in the TRAMP pathway for RNA
decay [21]), this needs to be formally demonstrated in
SOXmediated decay The identity of the mRNA decay/
surveillance pathway that is being usurped by SOX is,
therefore, of great interest Third, it is unclear whether the
poly(A)+ mRNAs that are sequestered in the nucleus as a
consequence of SOX expression are cytoplasmic transcripts
relocalized via PABPC1 or whether they are nascent
mRNAs that accumulate as a result of a SOXinduced block
in nuclear mRNA export Answers to these and other
questions will assuredly provide greater insight into our
understanding of herpesvirus biology and cellular mRNA
decay/surveillance mechanisms
Acknowledgements
Studies on viral mRNA decay in the Wilusz laboratory are supported
by NIH grant AI63434
References
1 Garneau NL, Wilusz J, Wilusz CJ: The highways and byways
of mRNA decay Nat Rev Mol Cell Biol 2007, 8:113-126.
2 Lee YJ, Glaunsinger BA: Aberrant herpesvirus-induced
polyadenylation correlates with cellular messenger RNA
destruction PLoS Biol 2009, 7:e1000107.
3 Glaunsinger BA, Ganem D: Lytic KSHV infection inhibits
host gene expression by accelerating global mRNA
turno-ver Mol Cell 2004, 13:713-723.
4 Korom M, Wylie KM, Morrison LA: Selective ablation of virion
host shutoff protein RNase activity attenuates herpes
simplex virus 2 in mice J Virol 2008, 82:3642-3653.
5 Glaunsinger B, Chavez L, Ganem D: The exonuclease and
host shutoff functions of the SOX protein of Kaposi’s
sar-coma-associated herpesvirus are genetically separable J
Virol 2005, 79:7396-7401.
6 Kwak JE, Wickens M: A family of poly(U) polymerases RNA
2007, 13:860-867.
7 Küehn U, Güendel M, Knoth A, Kerwitz Y, Rüedel S, Wahle E:
Poly(A) tail length is controlled by the nuclear poly(A)
binding protein regulating the interaction between poly(A)
polymerase and the cleavage and polyadenylation
specifi-city factor J Biol Chem, in press.
8 Arias C, Walsh D, Harbell J, Wilson AC, Mohr I: Activation of
host translational control pathways by a viral
developmen-tal switch PLoS Pathog 2009, 5:e1000334.
9 Houseley J, Tollervey D: The nuclear RNA surveillance
machinery: the link between ncRNAs and genome
struc-ture in budding yeast? Biochim Biophys Acta 2008, 1779:
239-246
10 Anderson JT, Wang X: Nuclear RNA surveillance: no sign of
substrates tailing off Crit Rev Biochem Mol Biol 2009, 44:16-24.
11 Slomovic, S, Laufer, D, Geiger, D, Schuster, G: Poly
adeny-lation of ribosomal RNA in human cells Nucleic Acids Res
2006, 34:2966-2975.
12 West S, Gromak N, Norbury CJ, Proudfoot NJ: Adenylation and exosome-mediated degradation of cotranscriptionally
cleaved pre-messenger RNA in human cells Mol Cell 2006,
21: 437-443.
13 Roy G, De Crescenzo G, Khaleghpour K, Kahvejian A, O’Connor-McCourt M, Sonenberg N: Paip1 interacts with poly(A) binding protein through two independent binding
motifs Mol Cell Biol 2002, 22:3769-3782.
14 Mangus DA, Evans MC, Jacobson A: Poly(A)-binding pro-teins: multifunctional scaffolds for the post-transcriptional
control of gene expression Genome Biol 2003, 4:223.
15 Kieft JS: Viral IRES RNA structures and ribosome
interac-tions Trends Biochem Sci 2008, 33:274-283.
16 Conrad NK, Mili S, Marshall EL, Shu MD, Steitz JA:
Identification of a rapid mammalian deadenylation-depend-ent decay pathway and its inhibition by a viral RNA
element Mol Cell 2006, 24:943-953.
17 Palaniswamy V, Moraes KC, Wilusz CJ, Wilusz J:
Nucleophosmin is selectively deposited on mRNA during
polyadenylation Nat Struct Mol Biol 2006, 13:429-435.
18 Boyne JR, Colgan KJ, Whitehouse A: Recruitment of the complete hTREX complex is required for Kaposi’s sar-coma-associated herpesvirus intronless mRNA nuclear
export and virus replication PLoS Pathog 2008, 4: e1000194.
19 Mullen TE, Marzluff WF: Degradation of histone mRNA requires oligouridylation followed by decapping and simul-taneous degradation of the mRNA both 5’ to 3’ and 3’ to 5’
Genes Dev 2008, 22:50-65.
20 Rissland OS, Norbury CJ: Decapping is preceded by 3’
uri-dylation in a novel pathway of bulk mRNA turnover Nat
Struct Mol Biol, in press.
21 LaCava J, Houseley J, Saveanu C, Petfalski E, Thompson E, Jacquier A, Tollervey D: RNA degradation by the exosome is
promoted by a nuclear polyadenylation complex Cell 2005,
121: 713-724.
22 Joachims M, Van Breugel PC, Lloyd RE: Cleavage of poly(A)-binding protein by enterovirus proteases concurrent with
inhibition of translation in vitro J Virol 1999, 73:718-727.
23 Kuyumcu-Martinez M, Belliot G, Sosnovtsev SV, Chang KO, Green KY, Lloyd RE: Calicivirus 3C-like proteinase inhibits cellular translation by cleavage of poly(A)-binding protein
J Virol 2004, 78:8172-8182.
24 Alvarez E, Castelló A, Menéndez-Arias L, Carrasco L: HIV
pro-tease cleaves poly(A)-binding protein Biochem J 2006, 396:
219-226
25 Harb M, Becker MM, Vitour D, Baron CH, Vende P, Brown SC, Bolte S, Arold ST, Poncet D: Nuclear localization of cytoplas-mic poly(A)-binding protein upon rotavirus infection
involves the interaction of NSP3 with eIF4G and RoXaN J
Virol 2008, 82:11283-11293.
26 Ilkow CS, Mancinelli V, Beatch MD, Hobman TC: Rubella virus capsid protein interacts with poly(a)-binding protein and
inhibits translation J Virol 2008, 82:4284-4294.
27 Polacek C, Friebe P, Harris E: Poly(A)-binding protein binds
to the non-polyadenylated 3’ untranslated region of dengue
virus and modulates translation efficiency J Gen Virol
2009, 90:687-692.
Published: 11 August 2009 doi:10.1186/gb-2009-10-8-234
© 2009 BioMed Central Ltd